1. Field of the Invention
The present invention relates to energy trap type piezoelectric resonator components and, in particular, to an energy trap type piezoelectric resonator component having a structure that suppresses a fundamental wave of a thickness longitudinal vibration and that uses harmonics of the thickness longitudinal vibration.
2. Description of the Related Art
Energy trap type piezoelectric resonators are in widespread use as an oscillator or a resonator. A variety of vibration modes are selectively used depending on a target frequency.
Japanese Unexamined Patent Application Publication No. 11-168343 discloses an energy trap type piezoelectric resonator that uses a third overtone of a thickness longitudinal vibration mode. FIGS. 12 and 13 are respectively a perspective view and a plan view of the disclosed piezoelectric resonator 101.
A piezoelectric resonator 101 includes a parallelepiped piezoelectric substrate 102. Vibrating electrodes 103 and 104, each having a circular planar shape, are respectively arranged on the center of the top surface and the center of the bottom surface of the piezoelectric substrate 102. Extension electrodes 105 and 106 are respectively connected to the vibrating electrodes 103 and 104. The vibrating electrodes 103 and 104 face each other with the piezoelectric substrate 102 interposed therebetween. The piezoelectric substrate 102 is polarized in the direction of thickness.
If an alternating electric field is applied between the vibrating electrodes 103 and 104, the piezoelectric resonator 101 is vibrated in a thickness longitudinal vibration mode. The piezoelectric resonator 101 uses a third overtone of the thickness longitudinal vibration. Desired resonance characteristics are thus obtained.
When the thickness longitudinal vibration occurs, the fundamental wave thereof is also generated in addition to the third overtone. If the third overtone is desired, the fundamental wave becomes a spurious. For this reason, the line width of each of the extension electrodes 105 and 106 is selected in the piezoelectric resonator 101 to arrange a spurious control electrode for controlling the fundamental wave.
FIG. 14 is a plan view illustrating a piezoelectric resonator 111, using the third overtone of the thickness longitudinal vibration, disclosed in Japanese Unexamined Patent Application Publication 10-178329. The piezoelectric resonator 111 includes a parallelepiped piezoelectric substrate 112. A vibrating electrode 113 having an elliptical shape is disposed in the center of the top surface of the piezoelectric substrate 112. A vibrating electrode having an elliptical shape is arranged on the center of the bottom surface of the piezoelectric substrate 112. The vibrating electrodes 113 on both major surfaces of the piezoelectric substrate 112 face each other with the piezoelectric substrate 112 interposed therebetween.
Let L1 represent the major axis diameter of the vibrating electrode 113, and let L2 represent the minor axis diameter of the vibrating electrode 113, and the ratio of L1/L2 falls within a range of 1.10 to 1.75. An extension electrode 114 is connected to the elongated side of the vibrating electrode 113. The line width of the extension electrode 114 is 0.9 to 1.1 times the major axis diameter L1 of the vibrating electrode 113. Harmonics of expanding vibration different from the thickness longitudinal vibration are positively excited. The harmonics of the expanding vibration are superimposed on the fundamental wave of the thickness longitudinal vibration to suppress the fundamental wave.
In the piezoelectric resonator 101 disclosed in Japanese Unexamined Patent Application Publication No. 11-168343, a spurious control section is arranged in the extension electrodes 105 and 106. The fundamental wave is suppressed by intentionally enlarging the fundamental wave of the thickness longitudinal vibration in the spurious control section and then suppressing the fundamental wave in an encapsulation section.
In the piezoelectric resonator 111 disclosed in Japanese Unexamined Patent Application Publication 10-178329, the fundamental wave of the thickness longitudinal vibration is suppressed by superimposing the harmonics of the expanding vibration on the fundamental wave.
In each of these known devices, the area of the vibrating electrode is not increased in the structure for suppressing the fundamental wave of the thickness longitudinal vibration under the limitation that the third overtone in use is not suppressed. When an attempt is made to achieve a variety of frequency resonance characteristics by increasing the area of the vibrating electrode, the fundamental wave of the thickness longitudinal vibration is not sufficiently suppressed.